A standard drum centrifuge has a housing in which a foraminous drum is rotated at high speed about its axis. In starch manufacture, a suspension is fed to the interior of the drum so that it is thrown centrifugally against the wall thereof. At first the suspension forms an annular body in the drum having an inner surface centered on the axis, then this annular body stratifies and the liquid phase passes through the drum while the solid phase stays behind on the inner surface of the drum as a filter cake. The inner layer of liquid passes radially outward, under the influence of centrifugal force, through the layer of solids until the cake is substantially dry. As a rule, the drum is filled and refilled in a number of cycles until the filter cake builds up to a desired depth. This cake is then rinsed or washed by introducing wash liquid into contact with the internal surface of the cake so that the wash liquid passes radially outward through the cake under continued centrifugation until the cake is once more substantially dry. In some cases, a hot gas can be passed through it to further dry it, and finally it is physically stripped out of the drum and the cycle is restarted.
The co-current washing procedure described above has the disadvantage that, due to the well-compacted cake condition produced, the wash liquid moves through the cake very slowly, thus prolonging the time consumed in the wash process. It is also true that channeling of the wash may occur in the cake as a result of cake cracking; this leading to low washing efficiency by virtue of poor distribution of wash liquid in the cake. Further, this form of washing tends to increase compaction of both fine and large particles in the cake.
In the case of starch washing, this means that the fine protein (gluten) particles are retained in the cake with the larger starch particles, whereas it would be desirable to separate these particles and remove the gluten to obtain purified starch.
It is clear that there are problems in washing cake or slurry bodies produced by centrifugation which require attention and demand improvement.
The centrifuge filter (peeler centrifuge) is the subject of prior U.S. patents including U.S. Pat. No. 5,066,271, issued Nov. 19, 1991, and U.S. Pat. No. 5,091,084, issued Feb. 25, 1992 which disclose various aspects of this technology.